In a typical cellular radio system, a wireless terminal(s) communicates via a Radio Access Network (RAN) to one or more Core Networks (CN). The wireless terminal is also known as mobile station and/or User Equipment (UE), such as mobile telephone, cellular telephone, smart phone, tablet computer and laptop with wireless capability. The user equipment may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices which communicate voice and/or data via the RAN. In the following, the term user equipment is used when referring to the wireless terminal.
The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS), which in some networks is also called NodeB, B node, evolved Node B (eNB) or Base Transceiver Station (BTS). The term base station will be used in the following when referring to any of the above examples. A cell is a geographical area where radio coverage is provided by the base station at a base station site. The base station communicates over an air interface operating on radio frequencies with the user equipment within range of the base station.
In some versions of the RAN, several base stations are typically connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC). The RNC, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more CNs.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. Universal Terrestrial Radio Access Network (UTRAN) is essentially a RAN using WCDMA for user equipments. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based RAN technologies.
In a WCDMA network or similar a key component is to maintain a received Signal-to-Noise plus Interference Ratio (SINR) at a constant level to preserve the quality of the received information at a desired level. SINR is a measure of signal strength relative to background noise and interference. The base station receives a signal from the user equipment and measures the SINR of the received signal, then the measured SINR value is compared with a SINR threshold-value to generate a Transmission Power Control (TPC) command. The TPC command is sent to the user equipment and indicates to the user equipment whether it should increase or decrease its transmitting power. The user equipment adjusts its transmitting power based on the received TPC command. The adjustment may take place for example once for a time slot.
Transmit power control is important for a smooth operation of a WCDMA system or similar and it is used in order to keep the received information quality at a set level. In general, the base station sends a TPC command to the user equipment to adjust the power at the user equipment. Two fundamental methods are specified for this purpose: Outer loop power control and inner loop power control. The purpose of the inner loop power control is to minimize the power and interference of ongoing connections by keeping the received Signal to Interference-plus-Noise Ratio (SINR) at a target level. The purpose of the outer loop power control is to adjust the target SINR for the inner loop so that the resulted Block Error Rate (BLER) of the data blocks may meet a certain BLER target. BLER is a ratio of the number of erroneous blocks to the total number of received blocks. SINR is the ratio of the received strength of the desired signal to the received strength of undesired signals, noise and interference. SINR is calculated using the following equation
  SINR  =      P          I      +      N      where P is the signal power, I is the interference power and N is the noise power.
SINR is commonly used in wireless communication as a way to measure the quality of wireless connections. The energy of a signal fades with distance, which is defined by the path loss. A wireless communication network has to take a lot of parameters into account such as e.g. the background noise, interfering strength of other simultaneous transmission, and the SINR attempts to create a representation of this aspect.
If the user equipment 107 moves towards the base station 101, the signal strength increases and causes increased interference level as seen by other user equipment's. In this case, the base station 101 needs to send an instruction to the user equipment 107 to reduce its transmission power as it moves towards the base station 101. If the user equipment 107 moves away from the base station 101 it will suffer from increased path loss. In this case, the base station 101 needs to send an instruction to the user equipment 107 to increase its transmission power as it moves away from the base station 101.
According to some embodiments of 3GPP the user equipment's transmission power should be updated each 0.667 ms based on a signal quality measurement done in the base station. 3GPP describes two different inner loop power control algorithms. A schematic figure of the inner loop power control is presented in FIG. 1. In FIG. 1 the base station 101 sends an UpLink (UL) TPC command 103 to the user equipment 107 comprising instructions to increase or decrease its transmission power. The user equipment 107 adjusts its transmission power according to the TPC command and sends a signal according to the adjusted transmission power, TX power 108, back to the base station 101. Uplink is defined as the direction from the user equipment 107 to the base station 101, and downlink is defined as the direction from the base station 101 to the user equipment 107.
In algorithm 1 for inner loop power control, every TPC command is handled individually resulting in 1 dB user equipment Dedicated Physical Control CHannel (DPCCH) transmit power increase/decrease every slot. For example, in WCDMA, the increase/decrease is +/−1 dB. IN LTE, it may be other values, such as e.g. +2, +1, 0, −1.
The transmit powers of other physical channels are set relatively to the DPCCH channel with a corresponding predefined power offset. Hence the transmit power of other physical channels are also adjusted in the same scale when the DPCCH power is adjusted. DPCCH is the physical channel on which the control signaling is transmitted, both on the uplink by the user equipment 107 to the base station 101 and on the downlink by the base station 101 to the user equipment 107.
According to algorithm 1, when the user equipment 107 is not in soft handover, i.e. it is only connected to one cell during a call, only one TPC command will be received from the base station 101 in each slot, in which a TPC command is known to be present. In this case, the value of TPC command is derived as follows:    if the received TPC command is equal to 0 then TPC command for that slot is −1,    if the received TPC command is equal to 1, then TPC command for that slot is +1.
In algorithm 2 for inner loop power control, five consecutive TPC commands must be recognized as increase to generate one 1 dB up, or five consecutive TPC commands must be recognized as decrease to generate one 1 dB down. When the user equipment 107 is not in soft handover, only one TPC command will be received in each slot. In this case, the user equipment 107 shall process received TPC commands on a 7-slot cycle, where the sets of 7 slots shall be aligned to the frame boundaries and there shall be no overlap between each set of 7 slots. The value of TPC command is derived as follows:    For the first 4 slots of a set, TPC command=0.    For the 7 slot of a set, the user equipment 107 uses hard decisions on each of the 7 received TPC commands as follows: (1) If all 7 hard decisions within a set are 1, then TPC command is set to +1 in the fifth slot; (2) If all 7 hard decisions within a set are 0, then TPC command is set to −1 in the fifth slot; (3) Otherwise, the TPC command is set to 0 in the fifth slot.
The hard decisions mentioned above are related to binary information and decides whether a received bit is a one or zero.
The basic idea with the 3GPP inner loop power control is to combat the user equipment's 107 own channel variations and keep the signal quality on a predefined level. The predefined level may be seen as a target quality value. The target may be predefined or semi-static to its nature.
For example, the block error rate (BLER) may be chosen to be defined as the measure of signal quality. A target of e.g. BLER=10% may be set as the predefined value towards which the system performance may be steered, using the TPC commands.
Algorithm 1 above suits normal speech user equipment's 107 quite well if they are transmitting on power levels well below the thermal noise. But with the introduction of Enhanced UpLink (EUL), user equipment's 107 transmitting with high data rate on the uplink, the power level from individual user equipment's 107 might reach above the thermal noise and interfere with other transmitting user equipment's 107 in the network. EUL provides high data rates capacity for packet data services on the uplink. In a multi-user equipment scenario it is in most cases more important to avoid creating interference to other user equipment's than to combat the user equipment's 107 own channel variations.
The base station 101 issues the TPC command to adjust the power at the user equipment 107. However, the adjustment takes place after some delay, referred to as the TPC delay. This delay is typically the propagation and the processing time at the user equipment 107 and the base station 101. The processing time may be for example the SINR estimation time in the base station 101 and/or the TPC command decoding time in the user equipment 107.
The TPC delay may also be described as the time duration from the uplink slot. For a short Transmission Time Interval (TTI) such as e.g. 2 ms TTI, the TPC delay based on the measured SINR of which the TPC is generated, to the user equipment 107 transmits an uplink slot with an adjusted power with the said TPC command. This may cause algorithm 1 to over control the network as the response time of the network due to the TPC delay is comparable to or even longer than the TTI. TTI refers to the duration of a transmission on the radio link.
The negative impact on the system and user equipment performance is larger for a larger TPC delay, e.g. D=2 slots, which is especially essential for some advanced receivers such as Serial Interference Cancellation (SIC) or Parallel Interference Cancellation (PIC) receivers. The letter D represents the delay. See FIG. 2 for illustration of TPC delay. FIG. 2 shows the TPC commands generated by the base station 101. In this example, three TPC commands are illustrated, TPC n, TPC n+1 and TPC n+2, with the respective base station Rx timing n, n+1 and n+2, where n is a positive integer. The user equipment 107 receiving the TPC commands is also illustrated in FIG. 2. The user equipment's 101 Tx timing is n+D, n+D+1 and n+D+2 for the corresponding to the applied TPCs, TPC n, TPC n+1 and TPC n+2. As mentioned above, the D is the TPC delay and D is an integer number of slots, such as for example 0, 1, 2, 3, . . . .
The TPC delay may typically depend on the processing time needed for SINR estimation, uplink/downlink timing and propagation distance between the user equipment and the base station. With future advanced receivers such as interference cancellation receivers the processing time may be even higher and thus the TPC delay may be as high as 8 time slots.
Algorithm 2 above is expected to be very slow, has problems to follow a fading channel and is quite sensitive to SINR estimation errors. For instance, the user equipment 107 has high risk of suffering from radio link failure in case of sudden uplink quality degradation with Algorithm 2.
Once a dedicated channel is established, the inner and outer loop work together to maintain the required BLER. A switch between Algorithm 1 and 2 requires Radio Resource Control (RRC) signaling, which means a cost of both signaling and delay. The RRC protocol is being used to configure and control the radio resource between the base station 101 and the user equipment 107.